The present invention relates to an electronic distance measuring device in which a phase difference of an oscillating wave, such as a light wave, that has traveled along two different light paths is used to measure a distance from the device to an objective station.
FIG. 1 shows a conventional electronic distance measuring device disclosed in U.S. Pat. No. 4,636,068.
A modulated light beam emitted from a light source 31 is reflected by a prism 33 through an objective lens 34 to be incident on a reflector 35 that is located at an objective station. The light is reflected by the reflector 35, transmitted towards the objective lens 34 and reflected by the prism 33 to be incident on a detector 42. The electronic distance measuring device measures a distance from the objective station by detecting a phase difference between the phase of the reflected light from the reflector 35 and the phase of the emitted light.
In the electronic distance measuring device in which the phase difference measuring method is employed, the accuracy of the distance measurement depends on the accuracy of the detection of the phase difference. Therefore, the accuracy of the detection of the phase difference must be very high in order to accurately measure the distance. In general, the order of the accuracy to detect the phase difference is approximately 10.sup.-7 second.
In order to accurately detect the phase difference, the electronic distance measuring device is required to discriminate a regular portion of the reflected light, which corresponds to the transmitted modulated light, from an irregular noise portion.
In general, a second order reflection light component of the irregular noise portion will disturb the discrimination of the regular portion of the reflected light more than the other irregular noise components. The second order reflection light component is defined as stray light that is generated when the regular modulated light is reflected by a high-reflectivity surface located in the light transmitting path 36A between the light source 31 and the objective lens 34, or in the light detecting path 36B between the objective lens 34 and the detector 42.
For instance, in the light transmitting path 36A, the second order reflection light component is generated when a portion of the light (light 41a) that is propagated along the light paths 36A and 36B is reflected by a high-reflectivity surface such as the detector 42. The reflected light (i.e., light 41a) is then propagated in the reverse direction along the light paths 36B and 36A, towards the light source 31. However, at the high-reflectivity surface of the light source 31, a portion 41b of the reflected light 41a is reflected back along the light paths 36A and 36B and is detected by the detector 42.
In order to solve this problem, the conventional electronic distance measuring device shown in FIG. 1 reduces the second order reflection light component by using two attenuating filters. One of the filters is mounted on shutter plate 38 located between the light source 31 and the prism 33. The other filter is a variable attenuation filter 40 located between the prism 33 and the detector 42.
The shutter plate 38, rotated by a motor 32, has two slits to divide the light emitted from the light source 31 into two portions. One portion of the emitted light is propagated along the light transmitting path 36A and the other portion is propagated along reference light path 37. A neutral density (hereinafter referred to as ND) filter 39 is placed in the slit of the shutter plate 38 in order to attenuate the light that is propagated along the light transmitting path 36A.
The variable attenuation filter 40 is formed as a circular disk. An amount of light transmitted through the circular disk varies along a circumferential direction thereof. A motor 43 rotates the variable attenuation filter 40 in order to change the amount of light transmitted therethrough.
The regular reflection light is transmitted through each of the attenuating filters twice along the optical path from the light source 31 to the detector 42. However, the second order reflection light component is transmitted through the attenuating filters at least four times along the optical path from the light source 31 to the detector 42. Accordingly, the intensity of the second order reflection light component at the detector 42 is much lower than the intensity of the regular reflection light.
However, in the electronic distance measuring device shown in FIG. 1, the ND filters become new sources for generating stray light, because the surfaces of the filters are also high-reflection surfaces. Therefore, it is not possible to increase the signal-to-noise (S/N) ratio. Accordingly, the accuracy of the detection of the phase difference is not very high.
Further, since the ND filter 39 is formed as an interference film by a vacuum evaporation method, it is difficult to reduce the reflectivity of the surface. Furthermore, since the ND filter of the variable attenuation filter 40 is formed as a glass plate, an anti-reflection film is required to reduce the reflection. This will increase the cost of manufacturing the ND filter, and therefore, the cost of manufacturing the electronic distance measuring device.